I used to be a professional evolutionary psychologist, researching and teaching the subject for more than 20 years. I was pretty careful to state my assumptions - which was that Ev Psych was a model of human behaviour that assumed the correctness of Natural Selection as a major influence on human psychology. It was not the only, nor necessarily the most important influence - and in many situations it was not relevant.
Yet, although careful to state these assumptions, there was a strong tendency to assume that Ev Psych was indeed the primary influence on human behaviour. In other words, discoursing on the subject tended to convert an assumption into a personal motivation.
More accurately, from a combination of habit and expertise, I tended to regard what was actually an assumption as a discovery; converting what was assumed into an apparent fact about the world.
This type of thing is rampant in the modern world - indeed defines the modern world (called positivism, scientism, reductionism, materialism...). Because I was deep into it, and came out from it (mostly...) I am perhaps able to see this with more clarity than most - I have been sensitised to it.
So, what about sex and relationships? There is a pseudo-biological, bastardised Ev Psych that dominates most discussion. I can see that there is an assumption in mainstream public discourse - and in analytic writings on the subject, that human sexuality is about having sex (especially with attractive people).
From a biological perspective this is a plain error, since Ev Psych is about having babies - and the link between having sex and having babies is so broken (by contraception and abortion) that there are plenty of people with more marriages than children; and indeed in some surveys the most children (on average) come from people who are once married, and have had one lifetime sexual partner.
There is a biological error - but the problem is much deeper than that because we should not be regarding ourselves a reproducing organisms, as if that was (or should be) our prime consideration.
The irony is that men reproduced successfully when they were primarily religious, and as soon as they began to regard themselves as 'higher animals' with biological goals, average reproduction slipped below minimum replacement levels, and such populations first began to age, and more recently become genetically degraded and move toward extinction.
Yet this discourse is still a grossly simplified,hence distorted model. It is clear by their actual behaviours ('revealed preferences') that people do not really regard themselves biologically, and show no sign ever of doing so. Indeed unnatural, non-reproductive, forms of sex and sexuality are being officially encouraged at global, national and local levels. So biology has about as little current relevance (except as an unavoidable constraint, dishonestly denied) as is imaginable.
How, then, ought people to talk about sexual relationships?
Well, first they must be clear with themselves what is their primary motivation. Someone who wants to marry one person for life and raise a family has a totally different perspective, hence attitudes and behaviours; from one who is motivated to have sex-without-strings with many attractive people; perhaps ideally a new person each week, or every few days...
The difference is spiritual, ultimately - it is the difference in a person's vision of what life should be; and this is based on an understanding of what life actually is.
If people were coherent, this would lead them straight back to First Things; to a discovery and evaluation of their own primary assumptions about 'Life, The Universe and Everything' - for example whether this world is created or accidental, whether it is personal or abstract and so forth.
But people are not coherent, indeed most people are psychotic in their denials and delusions. And this is a consequence of having chosen to live by the implicit mainstream assumption that human life, at its root/ depth/ the bottom-line, is 'about' no more than personal feelings.
It ought to be obvious that if our assumptions are that human life is a kind of emotional garbage; then we will lead our life on that basis. If we regard ourselves as disposable trash, and talk that way - to others in public and our-selves in private - we will become garbage, 'human resources'... to be burned, deployed as bio-landfill, or dismembered for salvage and recycling.
If we see life as a maximisation of pleasure and minimisation of suffering - we will regard sexual relations as an endless war of attempted manipulation and exploitation of other people to that end.
There will be a few winners, and many losers - but (because we are talking about pleasure) all 'victories' will be temporary, and habituation will mean that repeated pleasures lose effect. Everyone will be a loser sooner or later - victory is merely to be a winner now.
But everybody, without exception, loses in the end: which is death.